> Taldi Leifur nú mjög á hendur förunautum sínum
> Leif reproached his fellow travelers
much (I wasn't sure whether hendur was from handa the verb or meant at
the hands of)

It's their hands. I suppose the idiom comes from the idea that people
often do things with their hands, so blaming someone's hands became a
round about way of blaming the person themselves. (Compare: 'njótið
heilir handa' "bless your hands", i.e. "well done!"

> og skal sinn dag hvort, lesa vínber eða höggva vínvið og
fella mörkina

"and we'll alterate the work, spending one day gathering grapes, the
next cutting vines and felling [wood from] the forest" (day 1, grapes;
day 2, cutting; day 3 grapes, and so on). Compare: 'og var sinn vetur
hvort, utan lands eða með föður sínum' "and he spent the winters
alternately abroad and with his father" (one winter abroad, the next
with his father, and so on).

> Þá tók einn maður til máls og mælti við Leif: "Hví stýrir þú svo
mjög undir veður skipinu?" Leifur svaraði: "Eg hygg að stjórn minni en
þó enn að fleira. Eða hvað sjáið þér til tíðinda?"

"Then one man spoke up and said to Leifr, 'Why are you sailing so
close to the wind?' Leifr answered, 'I'm attending to my steering and
to more besides. What can you see that's of note?'"

It'll make more sense when we get the rest of the conversation. The
sailer implies that Leifr isn't paying attention to where he's going.
Leifr insists that his mind is on the job, but that he's attending to
more than that. To demonstrate that he really is on the ball, Leifr
asks them if they can see anything out of the ordinary. They say not.
He goes on to demonstrate his superior attentiveness, having a little
fun at the crew's expense...